248 A. A. C. ELIOT MERLIN ON NAVICULA SMITHII AND N. CRABRO. 



arranged in oblique double rows, precisely like the " pearls " 

 shown in the photograph taken by our President. I should here 

 clearly state, however, that the form photographed seems to be 

 the rarer and much smaller elliptical species also mentioned by 

 Van Heurck, and shown in Plate B, fig. 23 of the Supplement 

 to his work. I have not examined this variety, and its minute 

 structure is not necessarily identical with that of the large kind 

 in my collection. 



It must also be pointed out that all the species described 

 by Van Heurck are apparently Belgian forms, and, besides being 

 smaller, may not be exactly similar in construction, or at least 

 more elusive and difficult, than the Naples specimens ; but even 

 should it prove that all possess analogous perforated caps 

 crowning the primaries, it only shows how easily omissions may 

 arise in the interpretation of detail, the exact nature of which 

 has been supposititiously elucidated by high authorities pre- 

 sumably employing objectives of exceptional excellence, and 

 points to the absolute necessity for rigidly critical and careful 

 methods of research, in order to arrive at a true conception of the 

 ultimate structure of minute objects so far as our present optical 

 appliances will allow. 



I would strongly recommend any one interested in the subject 

 to obtain a good strewn slide of the various varieties of Navicula 

 8mithii, mounted in styrax, and to examine it with a properly 

 corrected and adjusted x^-in. oil-immersion objective, the full 

 cone of a dry aplanatic condenser, and Gifford's screen. A light 

 form of stand may prove sufficient in this case for the coarser 

 examples, although for really difficult faint diatomic images I 

 have found the Powell No. 1 to be absolutely necessary. 



Addendum to Note on Navicula Smithil 

 Since writing the above a small valve, measuring about '07 mm. 



